Coherent optical communication systems usually employ digital amplitude and phase modulation to encode the data onto the carrier waveform using a Mach-Zehnder (MZ) modulator. The MZ modulator is intrinsically non-linear. Consequently, a first order distortion is normally applied to remove the basic non-linearity in the modulator transmission response for higher order modulation formats employing more than 2 modulation levels, such as 16 quadrature amplitude modulation (QAM). However, the performance can still be poor as a result of un-equal openings in the multiple level “eye” constellation associated with the received optical field.
Furthermore, the finite extinction ratio (ER) of the MZ modulator, due to an un-equal splitting ratio or un-equal optical losses, leads to modulation of the quadrature tributary, when the intention is only to modulate the in-phase tributary, and vice versa, for higher order modulation formats. In addition, un-equal electro-optic efficiencies for the two arms of the MZ modulator, also lead to modulation of the quadrature tributary, when the intention is only to modulate the in-phase tributary, and vice versa, for all modulation formats.